Once semiconductor integrated circuits became practical in the late 1960s, transistors took over from vacuum tubes in almost all areas except very high power. Major reasons for this were the requirement of a permanently powered hot filament as a source of electrons, the need to reduce the device's size for operation at high frequencies, and a technology that did not lend itself to extensive miniaturization.
In response to the industry's need for a more efficient display technology than liquid crystals or light emitting diodes, extensive work has been done on the phenomenon of cold cathode emission. Cold cathode, or field emission, devices are based on the phenomenon of high field emission wherein electrons can be emitted into a vacuum from a room temperature source if the local electric field at the surface in question is high enough. The creation of such high local electric fields does not necessarily require the application of very high voltage, provided the emitting surface has a sufficiently small radius of curvature.
Usually, cold cathode field emission devices are mass produced as an array of very small conical emitters, each of which is connected to a source of negative voltage via a cathode conductor line. Another set of conductive lines (called gate lines) is located a short distance above the cathode lines, intersecting with them at the locations of the conical emitters or microtips, and connected to a source of positive voltage. Both the cathode and the gate line that relate to a particular microtip must be activated before there will be sufficient voltage to cause cold cathode emission.
The electrons that are emitted by the cold cathodes accelerate past openings in the gate lines and are collected at an anode surface. In displays, the anodes are made of a luminescent material but if the anode is simply a collector, the device becomes a triode vacuum tube. Note that, even though the local electric field in the immediate vicinity of a microtip is in excess of 10 million volts/cm., the externally applied voltage is only of the order of several volts.
In a display, the above described components would normally be housed in a flat, vacuum tight, structure consisting of a front anode plate, a rear plate that bears the microtips as well as the power (cathode and gate) lines, separated from one another by side supports, located at their edges. Spacers, located between groups of microtips, are also often provided for added strength.
When field emission devices are to be used as vacuum tubes, the package described above is no longer needed since each micro vacuum tube can be individually sealed in its own vacuum environment. Nor need they be laid out in array form if other layouts are more efficient.
Micro vacuum tubes, of the general type described above, offer a number of advantages over transistors. These include:
there is zero leakage current when they are turned off (as opposed to the small, but finite, leakage of a back-biassed PN junction); PA1 they are not subject to hot electron effects which occur in MOS devices because of trapped charges; PA1 they are capable of operating at higher frequencies than transistors since there is no parasitic junction capacitance; and PA1 they have more linear IV curves than do MOS devices.
Before micro vacuum tubes can replace, or even co-exist with, transistors in integrated circuits, several problems still need to be overcome. These include accurate alignment of the microtip and the gate electrode, achievement of which would allow the use of narrower gate necks (offering better control), improving the emission efficiencies of the microtips, and making their manufacture fully compatible with integrated circuit manufacturing technology.
As will be described below, improved alignment of the emitting source relative to the gate electrode can be achieved by making the latter self-aligning. Before describing the improvements in emitter efficiency that result from the present invention, it is worth reviewing how microtips are currently manufactured. The most widely used method involves first forming a cavity that contains a cathode layer at one end and a gate electrode at the other, there being a small opening in the latter. By rotating this structure while a stream of evaporant is directed at it at an angle, a conical microtip is formed on the cathode layer inside the cavity, with its apex being level with the gate electrode opening. In practice the microtip and the opening are fused together and have to be etched back to separate them.
An alternative to the above method has been the use of a mold to form the microtip. Such a mold is readily formed if a conformal deposition method is used to partially fill a hole whose depth exceeds its diameter and that has curved walls--i.e. the diameter of the hole is a maximum at its top and a minimum at its bottom. As deposition proceeds, the diameter of the hole gradually decreases until the coatings on opposite sides at the bottom of the hole meet. If deposition is stopped before the hole fills up, a concave cusp will have formed inside the hole. When such a mold is filled with some other material a convex cusp, well suited to be a cold cathode emitter, is the result.
In macro technology, the next step would be the extraction of the molded part followed by its suitable placement. In micro technology this is not possible. Instead, the molded part is left in place and the mold is selectively etched away (sacrificed). Two key problems that must always be solved are how to access the mold (since it is often totally covered by the part) and how to leave the part firmly secured in the right place once the mold has been removed.
One solution to said key problems is presented by Zimmerman (U.S. Pat. No. 5,569,973 October 1996). Access holes to the sacrificial layer are etched through the microtip. These are located quite close to the actual tip to make sure that all sacrificial material near it gets removed. Etching of the sacrificial layer is very carefully controlled since all of it is not removed, the portion that is left behind after etching becoming the support for the microtip.
A third method for forming microtips is described by Bagley et al. (U.S. Pat. No. 5,266,530 November 1993). A pedestal is etched using a mask in the usual way except that substantial overetching is allowed to take place. This leads to severe undercutting of the pedestal's top, giving it the shape of a cone. Anodic etching is then used to further sharpen the tip. This method results in a self-aligned gate electrode but could not be used to form a structure having additional self-aligned gates.
Of interest also is Matsuzaki et al. (U.S. Pat. No. 5,576,986 November 1996) who describe a micro vacuum tube having a recess formed out of silicon oxide and a cold cathode emitter that has many combtooth-like tips.